Electric circuits
Current and Charge
Every component has its own agreed symbol.
A circuit diagram shows how components are connected together
A battery consists of 2 or more cells connected together
The size of an electric current is the rate of flow of charge
The equation for the electric current is: I = Q/t
Q = It
t = Q/I
Q = Charge = Coulomb = C
t = time = seonds = s
I = Current = Amps = A
Electrical Charges and Fields
Charges
Like charges repel
Unlike charges attract
Some insulating materials become charged when rubbed together
Electrons
+ively charged ions have lost electrons
-ively charged ions have gained electrons
Charged metal sphere
Positive = repelled
Potential Difference and Voltage
V = E/Q
Voltage(V) = Energy(J) / Charge(C)
R = V/I
Resistance(Ohms) = P.D(V) / Current(I)
Ohm's law
Current through a resistor
At a constant temperature
Directly proportional to the P.D acrodd the resistor
Reversing the P.D across a resistor reverses the current through it
IV graphs
Lamp
Diode
Resistor
Component Characteristics
Resistance = P.D / Current
Diode:
Forward resistance low.
Reverse resistance high.
A thermistor's resistance decreases as its temperature increases
Opposite to normal wires
An LDR's resistance decreases as the light increases
Required Practical
Find out if the resistance of components differ
Also see if reversing the circuit has any effect
Resistance of a wire is independent it
Plot your measurements on an IV graph
Series Circuits
In a series circuit, the same current passes through each component
In a series circuit, the total P.D of the power supply is shared betwen the components
The total P.D of cells in series is the sum of the P.D of each cell
Adding resistance is the total resistance
R(total) = R(1) + R(2) + ...
Adding more resistors in series increases the total resistance, and the total P.D is unchanged
Parallel Circuits
For Components in Parallel Circuits
The P.D across each component is the same
The total current is the sum of the currents through the separate components
The bigger the resistance of a component, the smaller the current that will pass through that component
Adding more resistors in parallel decreases the total resistance
Total P.D is unchanged
Total current through the resistors is increased